You can look at **[Omnifocus][1]** (a task management app), which has a repeat task option. 

They split it into two dropdowns once you select Repeat monthly.

**Option 1: select specific days for the month:**

[![enter image description here][2]][2]


**Option 2: select days of the week for each month**

[![enter image description here][3]][3]


###Update: Another option - make it read like a sentence###

You could also make the form to read like a sentence. 

[![enter image description here][4]][4]


  [1]: https://www.omnigroup.com/omnifocus
  [2]: https://i.sstatic.net/H6huB.png
  [3]: https://i.sstatic.net/313cq.png
  [4]: https://i.sstatic.net/IXuhl.png